bug: --shallow-since misbehaves on old branch heads

Hi everybody,

I think I have discovered a problem with clone/fetch --shallow-since: When a ref that is fetches has a head that is already older than the 'since' time, then the entire branch is fetched, instead of just the current commit.

Repro:

  rm -rf tmp out deep
  git init tmp
  for i in `seq 10 31`; do
      d="2017-05-${i}T12:00"
      GIT_AUTOR_DATE="$d" GIT_COMMITTER_DATE="$d" git -C tmp commit -m nix$i --allow-empty
  done
  git -C tmp checkout -b test HEAD^^^
  for i in `seq 10 14`; do
      d="2017-06-${i}T12:00"
      GIT_AUTHOR_DATE="$d" GIT_COMMITTER_DATE="$d" git -C tmp commit -m nax$i --allow-empty
  done
  for i in `seq 1 4`; do
      git -C tmp commit -m new$i --allow-empty
  done

  echo "** This is fine:"

  git clone --shallow-since '1 month ago' file://`pwd`/tmp out --branch test
  git -C out log --oneline

  echo "** This should show one commit but shows all:"

  git clone --shallow-since '1 month ago' file://`pwd`/tmp deep --branch master
  git -C deep log --oneline

Do I expect the wrong thing?

- Andreas